Abstract
While cumulative carbon dioxide (CO2) emissions dominate anthropogenic warming over centuries, temperatures over the coming decades are also strongly affected by short-lived climate pollutants (SLCPs), complicating the estimation of cumulative emission budgets for ambitious mitigation goals. Using conventional Global Warming Potentials (GWPs) to convert SLCPs to “CO2-equivalent” emissions misrepresents their impact on global temperature. Here we show that peak warming under a range of mitigation scenarios is determined by a linear combination of cumulative CO2 emissions to the time of peak warming and non-CO2 radiative forcing immediately prior to that time. This may be understood by expressing aggregate non-CO2 forcing as cumulative CO2 forcing-equivalent (CO2-fe) emissions. We show further that contributions to CO2-fe emissions are well approximated by a new usage of GWP, denoted GWP*, which relates cumulative CO2 emissions to date with the current rate of emission of SLCPs. GWP* accurately indicates the impact of emissions of both long-lived and short-lived pollutants on radiative forcing and temperatures over a wide range of timescales, including under ambitious mitigation when conventional GWPs fail. Measured by GWP*, implementing the Paris Agreement would reduce the expected rate of warming in 2030 by 28% relative to a No Policy scenario. Expressing mitigation efforts in terms of their impact on future cumulative emissions aggregated using GWP* would relate them directly to contributions to future warming, better informing both burden-sharing discussions and long-term policies and measures in pursuit of ambitious global temperature goals.
Generated Summary
This research explores an alternative approach to quantifying the contribution of non-CO2 emissions to future temperatures, focusing on ambitious mitigation scenarios. The study introduces a new usage of the Global Warming Potential (GWP), denoted as GWP*, to address the misrepresentation of CO2-equivalent emissions of short-lived climate pollutants (SLCPs). The methodology involves expressing aggregate non-CO2 forcing as cumulative CO2 forcing-equivalent (CO2-fe) emissions and using GWP* to relate cumulative CO2 emissions to the current rate of SLCP emissions. The research aims to provide a more accurate and scenario-independent approach to assessing the implications of future CO2 emissions, particularly in the context of ambitious mitigation goals. The study uses the MAGICC simple climate model and the RCP2.6 scenario to analyze the relationship between emissions and temperature changes. It also assesses the contributions of different regions to global temperature change using the GWP* metric. The study aims to provide a clearer understanding of how mitigation efforts impact future cumulative emissions and global warming, which will better inform burden-sharing discussions and long-term policies and measures to pursue ambitious global temperature goals.
Key Findings & Statistics
- The study uses the IPCC AR5 scenario database.
- The study mentions 2030 total CO2-e emission rates (GtCO2/yr)
- The study mentions categories of scenarios with AR5 categories (in ppm CO2eq) with the following values:
- Cat. 1: 430-480
- Cat. 2: 480-530
- Cat. 3: 530-580
- Cat. 4: 580-650
- The study mentions that in the scenarios, peak warming in scenarios is considered by Working Group 3 of the IPCC 5th Assessment Report (AR5).
- The study mentions that implementing the Paris Agreement would reduce the expected rate of warming in 2030 by 28% relative to a No Policy scenario, measured by GWP*.
- The study found that the residual spread of peak warming is 0.13 °C (standard deviation, or s.d.) about the best-fit line.
- Small dots show that almost all of this spread can be accounted for by differences in non-CO2 forcing between the different scenarios.
- The study mentions the conversion factor of 1274 GtCO2/(W/m²).
- The study shows the residual is significantly larger (s.d. of 0.052 °C) when cumulative CO2-e emissions computed using conventional GWPs are used to predict peak warming.
- The study mentions that the TCRE (Transient Climate Response to Emissions) model displays a TCRE of 1.69 °C/TtC for the 1000 GtCO2 emitted after 2005.
- The study mentions that the GWP100 values from AR5 indicate a central value of 1091 GtCO2/(W/m²), with a range of 866-1474 GtCO2/(W/m²).
- The study states that assuming nationally determined contribution (NDC) goals are met, and using the breakdown of emissions provided by ref. 8, combined CO2, methane and nitrous oxide emissions in 2030 are 28% lower than in a “Reference-No Policy” scenario if measured by GWP*.
- The same NDCs correspond to an 18% reduction in CO2-e emission based on GWP100.
- The study notes that the RMS fractional prediction error (FPE) is over 36% for GWP100.
- The study found that cumulative GWP100-based emissions (Fig. 3b) perform better, with an FPE of 9%.
- The study found that cumulative emissions based on GWP* (Fig. 3d) provide a very accurate indication of relative contributions to warming, with an FPE of only 2%.
- Annual GWP100-based emissions (Fig. 3a) are a relatively poor indicator of regional contributions to warming, with an FPE of 36.4%.
- The study mentions that, in the model used here, the AGWPH(CO2) decline dominates, such that CO2 emissions rise somewhat faster than CO2-induced warming after 2050 under a high (RCP8.5) emissions scenario.
- The study used the following values to calculate the 100-year integrated impulse response function: ro = 33.6 years; rc = 0.0206 years GtC⁻¹; r1 = 4.635 years K⁻¹
Other Important Findings
- The study demonstrates that peak warming under various mitigation scenarios is determined by a linear combination of cumulative CO2 emissions and non-CO2 radiative forcing.
- The study indicates that the contributions to CO2-fe emissions are well approximated by GWP*, which relates cumulative CO2 emissions with the current rate of SLCP emissions.
- The research shows that cumulative CO2-e emissions calculated using GWP100 are a poor indicator of peak warming and CO2-e emission rates are a poor indicator of temperature stabilization.
- The study emphasizes that cumulative emission budgets can be a useful tool for predicting peak warming under ambitious mitigation, provided non-CO2 forcings are accurately accounted for.
- The research suggests that CO2-fe emissions, which provide climatically equivalent emissions, can be used as a standard to assess other emission metrics.
- The study also reveals that under the GWP* metric, a sustained increase in SLCP emissions is equivalent to a negative sustained emission of CO2-e.
- The study also indicates that using GWP* to quantify regional contributions provides a clearer picture of how different regions contribute to global temperature change.
Limitations Noted in the Document
- The study relies on scenario-dependent assumptions about emissions after 2030, which may not fully capture the complexities of long-term emission scenarios.
- The study acknowledges that the correlation between total CO2-equivalent emissions and peak warming is weaker in the most ambitious scenarios.
- The study notes that the TCRE values may vary based on different temperature responses to different forcings in the MAGICC model.
- The study’s analysis is based on a specific version of the MAGICC model, and the results may vary with different climate models.
- The study mentions that the calculations for CO2-e and CO2-e* emissions use GWP100 values, which have uncertainties.
- The study acknowledges the challenges in accounting for regional contributions to global warming accurately due to diverse regional emission time-histories.
- The study acknowledges that the absolute magnitudes of CO2-fe emissions are affected by climate and carbon cycle uncertainties, but not their relative magnitudes or evolution over time.
Conclusion
The study underscores the importance of shifting from conventional GWP-based metrics to GWP* for a more accurate representation of the climate impact of emissions, particularly under ambitious mitigation scenarios. The use of GWP* offers a way to relate emissions to future warming directly, improving the transparency and effectiveness of global climate agreements. It also enhances the ability to assess the implications of policies and measures aimed at achieving global temperature goals. According to the study, the Paris Agreement’s goals can be better informed by relating mitigation efforts to their impact on future cumulative emissions, aggregated using GWP*. The conventional approach of assessing medium-term emission trajectories against long-term temperature goals is considered opaque and potentially misleading, highlighting the need for more precise metrics. The research suggests that NDCs and long-term strategies framed in terms of cumulative CO2-e* emissions would provide a more accurate picture of progress toward climate stabilization. The study posits that metrics that accurately reflect contributions to future warming will lead to greater transparency and better-informed global climate agreements. Ultimately, the study concludes that the GWP* metric is better suited for designing effective domestic policies and measures, making it a crucial step toward reducing ambiguity in climate outcomes and effectively implementing mitigation strategies to meet the goals of the Paris Agreement. This approach offers a more robust and transparent methodology for assessing regional contributions to global warming, and improving the formulation of emission metrics for climate change mitigation.